It is common in the field of medical examinations to assist in diagnosis to conduct various types of tests, whether conducted within a hospital environment, laboratory or physician's office. Such tests can range from simple patient observation to the use of complex examination and diagnostic equipment in which electrical stimuli are applied to a patient and the resulting electrical response signals from the patient are recorded, measured and analyzed. An electrocardiogram is an example of such an examination and diagnostic test in which electrical response signals from stimuli are recorded and analyzed. Electrocardiogram signals are indicative of a patient's heart condition and may be used to detect a heart attack or other cardiac condition. Another familiar medical diagnostic test is an electroencephalogram which uses stimuli to generate electrical signals from the brain of a patient which can be measured in the form of electrical potentials (referred to as evoked potentials) and which indicate the patient's brain activity. Data recorded from an electroencephalogram test are useful for determining such things as seizures or to assist the physician in the diagnosis of brain damage. Other examination procedures also use evoked potentials for diagnosing a variety of other diseases, including diseases of the central nervous system, auditory system and the visual system. Evoked potentials are typically determined by measuring electrical responses to sensory stimuli. When stimulation is applied to a particular sense of a human being, a corresponding brain potential is evoked at an information processing part of the brain that functions to manage the particular sense. Such evoked brain potentials are usually detected and measured by the use of electrodes positioned on the skin of the human head in the area of the information processing center of the brain corresponding to the particular sense involved.
Visual evoked potentials (VEP) are the evoked potentials in response to visual stimulation and are particularly useful to assist in diagnosing ophthalmic diseases in infants and young children, because such individuals are not always able to indicate responsiveness to visual stimuli or to verbalize the occurrence of vision failure. The retina contains more than 130 million light-receptor cells. These cells convert light into nerve impulses that are processed for certain features, which are transmitted by the optic nerve to the brain, where they are interpreted. Muscles attached to the eye control its movement. Birth defects, trauma from accidents, disease and age-related deterioration of the components of the eye can all contribute to eye disorders. Information processing in the brain is electrochemical in nature. Evoked potentials are the electrical responses of the brain elicited by sensory stimulation. The electrical responses of the brain produced by visual stimulation are visual evoked potentials. Changes in these visual evoked potentials can be used to pinpoint anomalies along the visual pathways. These visual pathways are interconnected linkages of cells, beginning with photoreceptor cells in the retina, passing through horizontal cells, bi-polar cells and amacrine cells to ganglion cells, which wind together to form optic nerve fibers leading to cells in the brain's thalamus which then leads to cells in the visual cortex. The retina does not register images and transmit them, unaltered, to the brain. Instead, it selects and abstracts biologically useful features of information in the patterns, which strike it, and transmits a selectively filtered message to the brain by means of interactions within and among neural networks. Further processing of the information then takes place in the brain by means of similar but more complex interactions in neural networks there. Anomalies in this electrical transmission are variations from the expected pattern in the reaction of cells along the visual pathways. They are believed to provide useful insight into many diseases and conditions affecting the brain, central nervous system, the eye and the ear. Therefore, one way to detect possible visual impairment in infants and small children is to record and measure visual evoked potentials in response to visual stimulation. Visual evoked potential analyzers can be used in screening for diseases and conditions of the brain, central nervous system, the eye and the ear. They detect abnormalities in the functioning of a patient's brain by analyzing the electrical responses of the brain, which occur when certain rapidly changing patterns of light displayed on a video screen, are viewed. These electrical responses are called potentials. Sensors attached non-invasively to the scalp permit measurements of visual evoked potentials and are widely used in basic research in vision and as an aid in the diagnoses of neurological and ophthalmic disorders. However, since these sensors record visual evoked potentials from large areas of the brain, relating changes in these recorded waves to specific neural processes has previously proven difficult or impossible.
Visual evoked potential systems have heretofore been used to test infant response to visual stimuli in order to determine the possible presence of amblyopia. Failure to detect amblyopia as early in life as possible could lead to incurable vision problems in adulthood. However, if detected early, amblyopia can be effectively treated. Accordingly, it has been found desirable to conduct visual evoked potential tests on infants and other humans. One such system, known as the VENUS System, was heretofore commercialized by Neuroscientific Corp. and is described in an article entitled “An Electrophysiological Technique for Assessment of the Development of Spatial Vision,” Optometry and Vision Science, Vol. 74, No. 9, Sept. 9, 1997.
Ear infections, or otitis media, are a major reason for doctor visits among preschoolers in the U.S., accounting for more than 24 million trips a year to the family physician. The problem is a serious one with the treatment of children under two years of age for such infections having tripled between 1975 and 1990. Children who suffer from repeated ear infections before age six often experience temporary hearing loss, speech and language delays, coordination difficulties and, in some cases, permanent hearing loss. Likewise many infants, about 4 in 1000 births, have hearing impairment problems that cause delays in speech, language and cognitive development. In many instances, hearing loss is not detected until the child is two to three years old and not speaking properly. The present invention can also be adapted to screen for malfunctions in hearing among infants and children thereby providing early detection for both eye and ear problems.
Visual evoked potentials are measured by detecting electrical signals from electrodes which are attached to the skin of the human patient while displaying visual stimuli to be observed by the patient. The electrical signals, representative of the visual evoked potentials generated in response to sensory perception of the visual stimuli, contain information about the visual sensory capability being processed along the visual neural pathways from the retina to the cortex of the brain. However, the visual evoked potential electrical signals are in the microvolt range, sometimes less than one microvolt, and are sometimes obscured by other random potentials arising in adjacent areas of the brain or as a result of random muscle movement of the head and neck or distraction of the subject. Accordingly, VEP is a signal that has a degree of variability and randomness that may also be a result of the test environment, poor electrode connection, or electromagnetic noise. Such variability and randomness could be of such significance as to render the measured visual evoked potentials useless. Accordingly, before visual evoked potential data is to be relied on for diagnosing a condition of the patient, it is important to determine if the recorded visual evoked potential data is reliable.